The invention relates to new sulfide derivatives of aromatic dithiocarboxylic acids, a process for their production and their use as nitrosamine-free crosslinking agents in rubbers for improvements in reversion stability. The polysulfide derivatives according to the invention have the general formula EQU (A--S--S--).sub.z X--S--S--A'
Crosslinking rubber with sulfur, or sulfur donors and accelerator systems, generally yields vulcanizates, whose polymer chains in the beginning phase are linked by polysulfide sulfur crosslinking. Polysulfide-linked vulcanizates with a great number of crosslinks lead to optimal results (cf. A. V. Chapman, M. Porter "Sulfur Vulcanization Chemistry" in "Natural Rubber Science and Technology," A. D. Roberts, ed. Oxford Press 1988). This applies to mechanical strength and especially to tear propagation resistance and also to abrasion resistance. But there is a disadvantage in such sulfur-containing systems insofar as they are subject to reversion which encompasses the decline of the vulcanizate properties during the vulcanization process and the anaerobic aging of the resultant vulcanized products during dynamic stress. The theories on the mechanism are that under thermal stress a breakdown of the polysulfide bridges to the monosulfide arrangement occurs. The sulfur thus being released is added to polymer segments and thus increases their glass transition temperature (Tg) but also catalyzes the attack of the oxygen on the vulcanizate. Both effects lead to damage of the crosslinks. These secondary reactions of the sulfur, which already begin during the vulcanization process as a function of the reaction time and reaction temperature, substantially affect the performance limits of the elastomers and therefore require limitation in view of the optimal configuration of the production process (see in this connection also A. V. Chapman, M. Porter in the above-cited article).
The use of the so-called EV systems represents an attempt to reduce the secondary reactions of the sulfur; they are system which consist of increased amounts of vulcanization accelerators with simultaneously minimal amounts of sulfur. But as a result of the faster prevulcanization, the processing reliability is adversely changed. There is added to this--and this is even more important--the preprogrammed impairing of the abrasion resistance, the tear propagation resistance and the cord adhesion, as is consistent with known tire technology (cf. P. M. Lewis "Vulcanisate structure and it effects on properties" in NR-Technologie, Quarterly Volume 17, Part 4, page 57ff., 1986).
To provide thermally stable vulcanizates, polysulfide derivatives were proposed (cf. DE-AS 22 65 382), which with the addition of thermal energy, release polysulfide segments. These polysulfide derivatives contain conventional accelerator systems, e.g., dithiocarbamoyl structures, as initial groups (A). But a considerable disadvantage of the dithiocarbamate system is that during the vulcanization process, nitrosamines are formed, which result by the nitrosation of the decomposition products (amines) of the dithiocarbamates unstable under these conditions. According to today's knowledge, nitrosamines are considered a health hazard, since they form mostly carcinogenic metabolites (cf. Umschau 1985 (1), 24). Amine-free polysulfide derivatives as crosslinking agents are also known, but they exhibit a completely unsatisfactory level of performance.